Amazon LightSail is a Virtual Private Server (VPS) that can be launched with a number of pre-built images such as WordPress. Think of it as a simpler version of EC2 with flat-rate pricing.
AWS CodeStar is a central hub for managing various interconnected services relating to software development, from writing code to deploying code.
The reason you couldn't find a comparison is that they are rather different services.
Basically, if you already have your code and just want to run it somewhere, then use EC2 or LightSail.
If you want to develop code using lots of AWS tools, use CodeStar.